A heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the organs and pump it into the lungs where the blood gets oxygenated. The body's metabolic need for oxygen increases with the body's physical activity level. The pumping functions are accomplished by contractions of the myocardium (heart muscles). An increase in the body's metabolic need for oxygen is satisfied primarily by a higher frequency of the contractions, i.e., a higher heart rate. In a normal heart, the sinoatrial node, the heart's natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to various regions of the heart to excite myocardial tissues in these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various regions of the heart to contract in synchrony such that the pumping functions are performed efficiently.
The functions of the sinoatrial node and the electrical conduction system are indicated by electrocardiography (ECG) with at least two electrodes placed in or about the heart to sense the action potentials. When the heart contracts irregularly or otherwise abnormally, one or more ECG signals indicate that contractions at various cardiac regions are chaotic and unsynchronized. Such conditions are known as cardiac arrhythmias. Cardiac arrhythmias result in a reduced pumping efficiency of the heart, and hence, diminished blood circulation.
Pacing therapy treats cardiac arrhythmias by using an implantable pacemaker to deliver electrical pulses that substitute for the action potentials to excite the myocardium, thereby restoring the functions of the sinoatrial note and/or the natural electrical conduction system. To ensure that the body receives sufficient oxygen to satisfy its metabolic needs, a pacing mode referred to as rate responsive pacing, or rate adaptive pacing, uses an indication of the body's physical activity level to dynamically adjust the pacing rate, which determines the frequency of the contractions.
Various mechanical functions of the heart, as well as electromechanical association between the electrical conduction system and the myocardium, are indicated by heart sounds. For example, amplitudes of the third heart sound (S3) and fourth heart sound (S4) are related to filing pressures of the left ventricle during diastole. Fundamental frequencies of S3 and S4 are related to ventricular stiffness and dimension. Chronic changes in S3 amplitude is correlated to left ventricular chamber stiffness and degree of restrictive filling. Change in the interval between atrial contraction and S4 is correlated to the changes in left ventricular end of diastolic pressure. Such parameters, being correlated to the heart's mechanical properties and electromechanical association, quantitatively indicate abnormal cardiac conditions such as heart failure, including degrees of severity, and need of appropriate therapies.
For these and other reasons, there is a need for an implantable pacemaker that senses the body's physical activity level and the heart sounds. Implantability requires that any circuit or functional module of the implantable pacemaker to be designed for the minimum size and energy consumption.